9 Capturing a Database Workload

9.2.1 Restarting the Database

While this step is not required, Oracle recommends that the database be restarted before capturing the workload to ensure that ongoing and dependent transactions are allowed to be completed or rolled back before the capture begins. If the database is not restarted before the capture begins, transactions that are in progress or have yet to be committed will not be fully captured in the workload. Ongoing transactions will thus not be replayed properly, because only the part of the transaction whose calls were captured will be replayed. This may result in undesired replay divergence when the workload is replayed. Any subsequent transactions with dependencies on the incomplete transactions may also generate errors during replay. On a busy system, it is normal to see some replay divergence, but the replay can still be used to perform meaningful analysis of a system change if the diverged calls do not make up a significant portion of the replay in terms of DB time and other such key attributes.

Before restarting the database, determine an appropriate time to shut down the production database before the workload capture when it is the least disruptive. For example, you may want to capture a workload that begins at 8:00 a.m. However, to avoid service interruption during normal business hours, you may not want to restart the database during this time. In this case, you should consider starting the workload capture at an earlier time, so that the database can be restarted at a time that is less disruptive.

Once the database is restarted, it is important to start the workload capture before any user sessions reconnect and start issuing any workload. Otherwise, transactions performed by these user sessions will not be replayed properly in subsequent database replays, because only the part of the transaction whose calls were executed after the workload capture is started will be replayed. To avoid this problem, consider restarting the database in RESTRICTED mode using STARTUP RESTRICT, which will only allow the SYS user to login and start the workload capture. By default, once the workload capture begins, any database instance that are in RESTRICTED mode will automatically switch to UNRESTRICTED mode, and normal operations can continue while the workload is being captured.

Only one workload capture can be performed at any given time. If you have a Oracle Real Application Clusters (Oracle RAC) configuration, workload capture is performed for the entire database. To enable a clean state before starting to capture the workload, all the instances need to be restarted.

To restart all instances in a Oracle RAC configuration before workload capture:

1. Shut down all the instances.

2. Restart one of the instances.

3. Start workload capture.

4. Restart the rest of the instances.

9.2.2 Using Filters with Workload Capture

By default, all user sessions are recorded during workload capture. You can use workload filters to specify which user sessions to include in or exclude from the workload during workload capture. There are two types of workload filters: inclusion filters and exclusion filters. You can use either inclusion filters or exclusion filters in a workload capture, but not both.

Inclusion filters enable you to specify user sessions that will be captured in the workload. This is useful if you want to capture only a subset of the database workload.

Exclusion filters enable you to specify user sessions that will not be captured in the workload. This is useful if you want to filter out session types that do not need to captured in the workload, such as those that monitor the infrastructure—like Oracle Enterprise Manager (EM) or Statspack—or other such processes that are already running on the test system. For example, if the system where the workload will be replayed is running EM, replaying captured EM sessions on the system will result in duplication of workload. In this case, you may want to use exclusion filters to filter out EM sessions.

9.2.3 Setting Up the Capture Directory

Determine the location and set up a directory where the captured workload will be stored. Before starting the workload capture, ensure that the directory is empty and has ample disk space to store the workload. If the directory runs out of disk space during a workload capture, the capture will stop. To estimate the amount of disk space that is required, you can run a test capture on your workload for a short duration (such as a few minutes) to extrapolate how much space you will need for a full capture. To avoid potential performance issues, you should also ensure that the target replay directory is mounted on a separate file system.

For Oracle RAC, consider using a shared file system. Alternatively, you can set up one capture directory path that resolves to separate physical directories on each instance, but you will need to consolidate the files created in each of these directories into a single directory. The entire content of the local capture directories on each instance (not only the capture files) must be copied to the shared directory before it can be used for preprocessing or data masking. For example, assume that you are:

• Running a Oracle RAC environment in Linux with two database instances named host1 and host2

• Using a capture directory object named CAPDIR that resolves to /$ORACLE_HOME/rdbms/capture on both instances

• Using a shared directory that resides in /nfs/rac_capture

You will need to login into each host and run the following command:

cp -r /$ORACLE_HOME/rdbms/capture/* /nfs/rac_capture

After this is done for both instances, the /nfs/rac_capture shared directory is ready to be preprocessed or masked.

9.6.1 Monitoring an Active Workload Capture

This section describes how to monitor an active workload capture using Enterprise Manager.

To monitor an active workload capture:

1. On the Software and Support page, under Real Application Testing, click Database Replay.

   The Database Replay page appears.

2. Under Active Capture and Replay, select the workload capture you want to monitor and click View.

   The View Workload Capture page appears.

   
   Description of the illustration dbr_capture_view.gif
   
   

3. Under Summary, information about the workload capture is displayed.

4. To view the workload profile, click the Workload Profile tab.

   Under Average Active Sessions, the Active Sessions chart provides a graphic display of the captured session activity compared to the uncaptured session activity (such as background activities or filtered sessions).

   Under Comparison, various statistics for the workload capture are displayed, including database time, average active sessions, user calls, transactions, session logins, and application errors. The statistics for the total session activity are displayed in the Total column, and the statistics for the captured session activity are displayed in the Capture column. The Percentage of Total column displays the percentage of total session activity that is being captured in the workload.

   To view the workload capture report, click View Workload Capture Report.

5. To view workload filters used by the workload capture, click the Workload Filters tab.

   Details about the workload filters used by the workload capture are displayed, including the workload filter name, type, session attribute, and value.

6. To return to the Database Replay page, click OK.

9.6.2 Stopping an Active Workload Capture

This section describes how to stop an active workload capture using Enterprise Manager.

To stop an active workload capture:

1. On the Software and Support page, under Real Application Testing, click Database Replay.

   The Database Replay page appears.

2. Under Active Capture and Replay, select the workload capture you want to stop and click Stop.

   The Confirmation page appears.

3. To confirm that you want to stop the workload capture, click Yes.

   Once the workload capture is stopped, the Export AWR Data page appears.

4. To export the Automatic Workload Repository (AWR) data, click Yes.

   The Export AWR Data page appears; click Yes.

   Exporting AWR data enables detailed analysis of the workload. This data is also required if you plan to run the Replay Compare Period report or the AWR Compare Period report on a pair of workload captures or replays.

   If you choose not to export AWR data, click No. You can still export AWR data from a completed workload capture at a later time from the View Workload Capture History page.

   The View Workload Capture page appears.

9.6.3 Managing a Completed Workload Capture

This section describes how to manage a completed workload capture using Enterprise Manager.

To manage a completed workload capture:

1. On the Software and Support page, under Real Application Testing, click Database Replay.

   The Database Replay page appears.

2. Click View Workload Capture History.

   The View Workload Capture History page appears.

   
   Description of the illustration dbr_capture_history.gif
   
   

3. To delete a workload capture, select the workload capture and click Delete.

   This will not remove the capture files from the capture directory.

4. To export AWR data for a workload capture, select the workload capture and click Export AWR Data.

   The Export AWR Data page appears; click Yes.

   Exporting AWR data enables detailed analysis of the workload. This data is also required if you plan to run the Replay Compare Period report or the AWR Compare Period report on a pair of workload captures or replays.

5. To view details about a workload capture, select the workload capture and click View.

   The View Workload Capture page appears.

6. Under Summary, information about the workload capture is displayed.

7. To view the workload profile, click the Workload Profile tab.

   Under Average Active Sessions, the Active Sessions chart provides a graphic display of the captured session activity compared to the uncaptured session activity (such as background activities or filtered sessions). This chart will be shown only when there is Active Session History (ASH) data available for the capture period.

   Under Comparison, various statistics for the workload capture are displayed, including database time, average active sessions, user calls, transactions, connects, and application errors. The statistics for the total session activity are displayed in the Total column, and the statistics for the captured session activity are displayed in the Capture column. The Percentage of Total column displays the percentage of total session activity that are being captured in the workload.

   To view the workload capture report, click View Workload Capture Report.

8. To view workload filters used by the workload capture, click the Workload Filters tab.

   Details about the workload filters used by the workload capture are displayed, including the workload filter name, type, session attribute, and value.

9. To return to the Database Replay page, click OK.

9.7.1 Defining Workload Capture Filters

This section describes how to add and remove workload capture filters. For information about using workload filters with workload capture, see "Using Filters with Workload Capture".

To add filters to a workload capture, use the ADD_FILTER procedure:

BEGIN
  DBMS_WORKLOAD_CAPTURE.ADD_FILTER (
                           fname => 'user_ichan',
                           fattribute => 'USER',
                           fvalue => 'ICHAN');
END;
/

In this example, the ADD_FILTER procedure adds a filter named user_ichan, which can be used to filter out all sessions belonging to the user name ICHAN.

The ADD_FILTER procedure in this example uses the following parameters:

• The fname required parameter specifies the name of the filter that will be added.

• The fattribute required parameter specifies the attribute on which the filter will be applied. Valid values include PROGRAM, MODULE, ACTION, SERVICE, INSTANCE_NUMBER, and USER.

• The fvalue required parameter specifies the value for the corresponding attribute on which the filter will be applied. It is possible to use wildcards such as % with some of the attributes, such as modules and actions.

To remove filters from a workload capture, use the DELETE_FILTER procedure:

BEGIN
  DBMS_WORKLOAD_CAPTURE.DELETE_FILTER (fname => 'user_ichan');
END;
/

In this example, the DELETE_FILTER procedure removes the filter named user_ichan from the workload capture.

The DELETE_FILTER procedure in this example uses the fname required parameter, which specifies the name of the filter to be removed. The DELETE_FILTER procedure will not remove filters that belong to completed captures; it only applies to filters of captures that have yet to start.

9.7.2 Starting a Workload Capture

Before starting a workload capture, you must first complete the prerequisites for capturing a database workload, as described in "Prerequisites for Capturing a Database Workload". You should also review the workload capture options, as described in "Workload Capture Options".

It is important to have a well-defined starting point for the workload so that the replay system can be restored to that point before initiating a replay of the captured workload. To have a well-defined starting point for the workload capture, it is preferable not to have any active user sessions when starting a workload capture. If active sessions perform ongoing transactions, those transactions will not be replayed properly in subsequent database replays, since only that part of the transaction whose calls were executed after the workload capture is started will be replayed. To avoid this problem, consider restarting the database in restricted mode using STARTUP RESTRICT before starting the workload capture. Once the workload capture begins, the database will automatically switch to unrestricted mode and normal operations can continue while the workload is being captured. For more information about restarting the database before capturing a workload, see "Restarting the Database".

To start the workload capture, use the START_CAPTURE procedure:

BEGIN
  DBMS_WORKLOAD_CAPTURE.START_CAPTURE (name => 'dec10_peak', 
                           dir => 'dec10',
                           duration => 600,
                           capture_sts => TRUE,
                           sts_cap_interval => 300);
END;
/

In this example, a workload named dec10_peak will be captured for 600 seconds and stored in the file system defined by the database directory object named dec10. A SQL tuning set will also be captured in parallel with the workload capture.

The START_CAPTURE procedure in this example uses the following parameters:

• The name required parameter specifies the name of the workload that will be captured.

• The dir required parameter specifies a directory object pointing to the directory where the captured workload will be stored.

• The duration parameter specifies the number of seconds before the workload capture will end. If a value is not specified, the workload capture will continue until the FINISH_CAPTURE procedure is called.

• The capture_sts parameter specifies whether to capture a SQL tuning set in parallel with the workload capture. If this parameter is set to TRUE, you can capture a SQL tuning set during workload capture, then capture another SQL tuning set during workload replay, and use SQL Performance Analyzer to compare the SQL tuning sets without having to re-execute the SQL statements. This enables you to obtain a SQL Performance Analyzer report and compare the SQL performance—before and after the change—while running Database Replay. You can also export the resulting SQL tuning set with its AWR data using the EXPORT_AWR procedure, as described in "Exporting AWR Data for Workload Capture".

  This feature is not supported for Oracle RAC. Workload capture filters that are defined using DBMS_WORKLOAD_CAPTURE do not apply to the SQL tuning set capture. The default value for this parameter is FALSE.

• The sts_cap_interval parameter specifies the duration of the SQL tuning set capture from the cursor cache in seconds. The default value is 300. Setting the value of this parameter below the default value may cause additional overhead with some workloads and is not recommended.

9.7.3 Stopping a Workload Capture

To stop the workload capture, use the FINISH_CAPTURE procedure:

BEGIN
  DBMS_WORKLOAD_CAPTURE.FINISH_CAPTURE (); 
END;
/

In this example, the FINISH_CAPTURE procedure finalizes the workload capture and returns the database to a normal state.

9.7.4 Exporting AWR Data for Workload Capture

Exporting AWR data enables detailed analysis of the workload. This data is also required if you plan to run the Replay Compare Period report or the AWR Compare Period report on a pair of workload captures or replays.

To export AWR data, use the EXPORT_AWR procedure:

BEGIN
  DBMS_WORKLOAD_CAPTURE.EXPORT_AWR (capture_id => 2);
END;
/

In this example, the AWR snapshots that correspond to the workload capture with a capture ID of 2 are exported, along with any SQL tuning set that may have been captured during workload capture.

The EXPORT_AWR procedure uses the capture_id required parameter, which specifies the ID of the capture whose AWR snapshots will be exported. This procedure will work only if the corresponding workload capture was performed in the current database and the AWR snapshots that correspond to the original capture time period are still available.